Decreased cerebral blood flow (CBF) is a significant problem seen in the most common and devastating brain disorders, including chronic hydrocephalus, stroke and dementia. Increasing clinical and experimental evidence indicate that the underlying cause in these and other neurological disorders symptomatic of decreased CBF stems from a loss of intracranial compliance (ICC).
In order for blood to enter the closed space of the rigid cranium, the brain must give way or be “compliant.” This idea is key to intracranial dynamics which involve the conduction of arterial blood pulse waves from the extracerebral arteries, via the cerebral spinal fluid (CSF), to the veins and spinal cavity, allowing the pulse waves to bypass the brain and its capillaries. Normally, arteries, when compliant, act as an elastic reservoir, the arterial walls absorbing part of the hydraulic energy in the pulse wave during systole. This energy is then released during diastole in order to maintain constant capillary flow. Termed the “Windkessel effect,” this process changes the pulse of arterial blood flow into a nearly continuous, non-pulsating capillary flow, decreasing the speed and force at which arterial pulse pressure is transmitted to the capillaries and the brain tissue in order to protect them from these forces.
However, in many brain disorders, intracranial hydrodynamics have become abnormal. Events that restrict arterial pulsations may cause these intracranial hydrodynamic abnormalities resulting in a decrease in intracranial compliance. For example, trauma and insults to and adhesions in the subarachnoid space can restrict arterial pulsations, causing a decrease in intracranial compliance. Similarly, any vascular disorder that results in increased capillary pulse pressure can lead to decreased ICC.
In brain disorders involving decreased intracranial compliance, arterial blood pulsations that reach the brain are poorly dampened or buffered, if at all. This breakdown in buffering by the Windkessel effect results in increased intracranial pulse pressure or pulsatility that acts directly on the brain, injuring the capillaries and brain tissue. In addition, decreased intracranial compliance causes increased vascular impedance, increased vascular resistance to convective blood flow (due to compressed arteries and veins) and, consequently, reduced cerebrovascular blood flow efficiency. In fact, these insults to the brain can account for the edema, brain thinning, decreased CBF and decreased brain function seen in vascular dementia, hydrocephalus, stroke and other neurologic diseases.
Traditionally, it has been believed that in brain disorders such as hydrocephalus, which is characterized by an increased volume of CSF in the brain, the pathology of the disease was due to an imbalance between CSF formation in the choriod plexus and CSF absorption in the capillaries of the central nervous system. Thus, one approach to treat hydrocephalus involved the draining or shunting of excess fluid from the CSF compartment to the subarachnoid space. However, shunting has not been effective in treating a number of patients and some now believe that the pathology of the disease may be due to a decrease in intracranial compliance, a problem shunting can not adequately address. Further, shunting is invasive, having the potential to damage and/or cause infection in the brain, is problematic in that the CSF is drained at non-physiological levels and is not a permanent solutions to hydrocephalus. Similarly, the treatments for other brain disorders like stroke or dementia do not address the reduced ICC seen in the diseases, which may, in fact, be the root cause of those diseases.
Moreover, there are numerous non-neurological diseases or conditions in which it would be advantageous to alter (e.g., increase or decrease) ICC, cerebral blood flow (CBF) or intracranial pressure (ICP) pulsatility/waveform. For example, conditions like vasospasms are characterized by abnormally high ICP pulsatility, while individuals who experience and/or have experienced congestive heart failure, caroitid endarterectomy or a cardiopulmonary bypass procedure are known to have undesirably low ICP pulsatility (see FIG. 1).
All of these diseases/conditions could benefit from a treatment that alters abnormal and/or undesirable ICC, CBF and ICP pulsatility/waveform. Thus, what is needed is a device that restores ICC by restoring the proper buffering of arterial pulsations in the brain, alters the flow of blood to the brain such that it is appropriate and/or modulates the ICP waveform such that the pulsatility is in a normal range.